Loran C is a radio navigation system operating in the low frequency portion of the radio frequency spectrum at a carrier frequency of 100 khz. The system utilizes chains which each consist of one master transmission station and two or more secondary transmission stations located at widely separated geographic locations. The master and secondary stations of each chain transmit at the same frequency but at different times. Each of the secondary stations transmits a series of eight closely and equally spaced radio frequency pulses, designated a pulse group, while the master station transmits a similar series of eight pulses followed by a delayed ninth pulse which is included for identification of the master station. Each master and secondary station continuously repeats transmission of its pulse group at a time spacing equal to the Group Repetition Interval (GRI) which is assigned to the chain to which the master or secondary station belongs. Typical GRI's range from about 40 to 100 milliseconds. The master station provides the time reference for all of the stations in its chain. Its pulse group is transmitted first, followed in time by pulse group transmission from the secondary stations in the chain in a selected order. The time of transmission of pulse groups by each of the secondary stations is selected so that there is no overlap in the reception of any of these pulse groups from the master or secondary stations by a receiver located anywhere within the nominal coverage area of the particular Loran C chain.
In the hyperbolic or range-difference mode of operation, the location or "fix" of a Loran C receiver within the coverage area of the chain is determined by synchronizing the receiver to the group repetition interval (GRI) at which the master and secondary stations of the particular chain are transmitting. The receiver calculates the difference between the time of arrival at the receiving station of the master station pulse group and each secondary station pulse group. For each pair consisting of a master station and a secondary station, the particular time difference will correspond to a hyperbolic line of position (LOP) on the surface of the earth which is a constant difference of distance between the master station and the particular secondary station, corresponding to a constant difference in the propagation time of the radio signal from the master station and the secondary stations. If the Loran C receiver can detect signals from a master and two secondary stations within a single chain, two lines of position may be determined by the receiver. The geographic point at which these two hyperbolic lines of position intersect provides an estimate of the location of the Loran C receiver.
In the circular geometric or direct-ranging mode, the location of a receiving station within the coverage area of a Loran C chain is determined by calculating the time of transmission of signals from stations. When these time differences have been calculated, the distances from the transmitting stations to the receiver can be computed. These distances correspond to radii of circles about the respective transmitting stations; the point of intersection of the three circles from the transmitting stations provides an estimate of the location of the receiver. To operate in this mode, it is generally necessary that the receiver have a highly stable and accurate master clock which can be synchronized to the pulse transmission times of the transmitters within the Loran C chain.
The radio signals available to the Loran C navigation receivers often have very low signal to noise ratios, making it difficult for the receiver to locate precise positions on each pulse waveform from the master and secondary stations with the accuracy needed to determine the exact time relationships between the stations. In addition, Loran C navigation is increasingly used in terrestrial and aeronautical applications in which the presence of various types of interfering radio frequency energy are more likely to be found than in the traditional maritime applications of Loran. These other sources of interference include power lines, commercial radio and television signals, and spurious radiation from many industrial and consumer products. Most of the interference from these types of sources are characterized by continuous wave transmission. The presence of continuous wave interference within the Loran C bandwidth may make the reception of useful Loran C navigation signal data difficult and sometimes impossible. The expansion of Loran C into non-marine applications and the subsequent construction of more Loran chains increases the potential for interference by transmission from neighboring chains, giving rise to cross rate interference (CRI), which is, of course, concentrated at the Loran C carrier frequency. In addition to the greater potential interference contaminating the Loran C signal in non-marine applications, such non-marine applications may require the receiver to make a measurement of position within a shorter time. The relatively slow movement of ships allows a relatively long period of time in which the Loran C receiver can acquire the signals and begin performing measurements. However, when Loran is being used for terrestrial navigation, and particularly for aircraft navigation, the significantly higher speeds at which the receiver is moving makes it essential that the receiver be able to rapidly acquire a signal and make a determination of position from the acquired signal information.
A Loran receiver utilizing an ensemble averaging technique to improve the signal to noise ratio of the received Loran signals is shown in the United States patent to Bahr, et al., U.S. Pat. No. 4,814,771. The Loran C signal of interest is periodically sampled to convert the continuously varying input signal to a stream of data representing the magnitude of the signal at the sample times. Each new data stream obtained during the current GRI is added to existing data in memory in a time-aligned fashion. This yields an ensemble averaged signal within the memory. In this manner, sporadic noise or non-synchronous energy, such as atmospheric noise, continous wave interference, and cross rate interference, is substantially reduced.
The effectiveness of the ensemble averaging operation is dependent on the system's ability to precisely time align successive data streams that are separated by a GRI period. A drift of 5 microseconds per longest GRI (or 50 Parts per million) between the receiver and transmitter clocks will cause Loran C pulses of alternate GRI's to be 180.degree. apart in phase. Pulses transmitted with the same phase in successive GRI's would add destructively in the averaging process.